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Command type: built-in commands (shell built-in)

External commands (there is an executable file corresponding to the command name under a path in the file system)

External command search depends on the PATH variable

View the external command search path and the PATH variable:

[[Email protected] ~] # Echo $ path

/Usr/lib64/qt-3.3/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin: /root/bin

(Paths are separated)

Type: displays the type of the specified command.

[[Email protected] ~] # Type Su

Su is/bin/SU (external command)

Which: shows the full path of (Shell) commadns to view the command path

Which [Options] [--] programname [...]

[[Email protected] ~] # Which Su

/Bin/su

Get Command help

1) whatis command

[[Email protected] ~] # Whatis Su
SU (1)-run a shell with substitute user and group IDs

(1) indicates a chapter

2) Help: displays brief summaries of builtin commands

Internal Command: Help Command

External command: Command -- Help

[[Email protected] ~] # Su -- Help
Usage: su [Option]... [-] [user [parameter]...]
Change the valid tive user ID and group ID to that of user.

3) MAN (manual command manual)

Man command

Man is generally divided into eight chapters:

(1) user commands

(2) system call

(3) database call

(4) special files (hardware files)

(5) file format (Configuration File Syntax)

(6) games

(7) Miscellaneous (Miscellaneous)

(8) management commands

Man screen flip:

Flip back: Space

One screen forward: B

Flip One Line Backward: Enter

One row forward: K

Man search:

/Keyword: Search backward

? Keyword: Forward search

N: Next

N: Previous

Q: Exit man.

[[Email protected] ~] # Man Su


SU (1) user commands SU (1)

Name
Su-run a shell with substitute user and group IDs

Synopsis
Su [Option]... [-] [user [Arg]...]

4) online document: info command

5) Documentation:/usr/share/doc

Exercise:

1) whether ECHO is an internal command or an external command

[[Email protected] ~] # Type echo
ECHO is a shell builtin

ECHO is an internal command

2) Role

Echo --- display a line of text

3) The line feed shows "123456. abcdef"

[[Email protected] ~] # Echo-e "123456 \ nabcdef"

123456

Abcdef

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